Single-atomic-site platinum steers photogenerated charge carrier lifetime of hematite nanoflakes for photoelectrochemical water splitting

Although much effort has been devoted to improving photoelectrochemical water splitting of hematite (α-Fe2O3) due to its high theoretical solar-to-hydrogen conversion efficiency of 15.5%, the low applied bias photon-to-current efficiency remains a huge challenge for practical applications. Herein, we introduce single platinum atom sites coordination with oxygen atom (Pt-O/Pt-O-Fe) sites into single crystalline α-Fe2O3 nanoflakes photoanodes (SAs Pt:Fe2O3-Ov). The single-atom Pt doping of α-Fe2O3 can induce few electron trapping sites, enhance carrier separation capability, and boost charge transfer lifetime in the bulk structure as well as improve charge carrier injection efficiency at the semiconductor/electrolyte interface. Further introduction of surface oxygen vacancies can suppress charge carrier recombination and promote surface reaction kinetics, especially at low potential. Accordingly, the optimum SAs Pt:Fe2O3-Ov photoanode exhibits the photoelectrochemical performance of 3.65 and 5.30 mA cm−2 at 1.23 and 1.5 VRHE, respectively, with an applied bias photon-to-current efficiency of 0.68% for the hematite-based photoanodes. This study opens an avenue for designing highly efficient atomic-level engineering on single crystalline semiconductors for feasible photoelectrochemical applications.

Although much effort has been devoted to improving photoelectrochemical water splitting of hematite (α-Fe 2 O 3 ) due to its high theoretical solar-tohydrogen conversion efficiency of 15.5%, the low applied bias photon-tocurrent efficiency remains a huge challenge for practical applications. Herein, we introduce single platinum atom sites coordination with oxygen atom (Pt-O/Pt-O-Fe) sites into single crystalline α-Fe 2 O 3 nanoflakes photoanodes (SAs Pt:Fe 2 O 3 -Ov). The single-atom Pt doping of α-Fe 2 O 3 can induce few electron trapping sites, enhance carrier separation capability, and boost charge transfer lifetime in the bulk structure as well as improve charge carrier injection efficiency at the semiconductor/electrolyte interface. Further introduction of surface oxygen vacancies can suppress charge carrier recombination and promote surface reaction kinetics, especially at low potential. Accordingly, the optimum SAs Pt:Fe 2 O 3 -Ov photoanode exhibits the photoelectrochemical performance of 3.65 and 5.30 mA cm −2 at 1.23 and 1.5 V RHE , respectively, with an applied bias photon-to-current efficiency of 0.68% for the hematite-based photoanodes. This study opens an avenue for designing highly efficient atomic-level engineering on single crystalline semiconductors for feasible photoelectrochemical applications. Photoelectrochemical (PEC) water splitting has attracted great promises in recent years for sustainable hydrogen production [1][2][3][4][5] , in which the fabrication of the suitable semiconductor photoanodes with sufficient light absorption and efficient charge carrier transport is becoming increasingly important for achieving high solar-to-hydrogen (STH) conversion efficiency [6][7][8][9][10] . Many metal oxide semiconductors such as TiO 2 [11][12][13] , α-Fe 2 O 3 [14][15][16] , BiVO 4 [17][18][19] , and WO 3 [20][21][22] , have been considered as promising candidates owing to their availability, facile preparation, and oxidative stability. Especially, hematite (α-Fe 2 O 3 ), a n-type semiconductor with a small band gap of~2.1 eV, can absorb a large portion of the solar spectrum and allow a theoretical STH efficiency of 15.5% under standard sunlight illumination [23][24][25][26] . However, its short hole diffusion length, poor charge carrier conductivity, and sluggish oxygen evolution reaction kinetics limit the photocurrent of α-Fe 2 O 3 far below its theoretical value of 12.4 mA cm −227, 28 . Especially, the low electron mobility (~10 −2 cm 2 V −1 s −1 ) as one of the main intrinsic drawbacks significantly impedes its PEC performance [29][30][31] .
To enhance the electron mobility of photoelectrode, doping elements (e.g., Ti [32][33][34] , Sn [35][36][37] , Zr 38 , La 39 , Ta 27,40 , B 41 , and P 31 ) have been adapted to substantially ameliorate the photo-efficiency of α-Fe 2 O 3 . For example, nonmetallic P doping has a superior activity owing to the strong covalent interaction between P and O, which boosted fast electron carriers and avoided the formation of deep electron trapping sites in α-Fe 2 O 3 31 . Although these doping elements can improve more or less the electrical conductivity and charge transfer of α-Fe 2 O 3 , all the dopants in the photoelectrodes reported so far are clusters or bigger than clusters. As a result, not only the improvement of PEC is limited, but also the band bending caused by these clusters decreases the width of space-charge layer which limits the number of the carriers inside of the layer. In addition, these traditional dopants have not much influence on onset potential (generally located at 0.8-1.0 V RHE ). The onset potential further influences the applied bias photon-tocurrent efficiency (ABPE) of α-Fe 2 O 3 due to the surface and bulk trap states 3,42 , the high onset potential causing to the low ABPE. Therefore, additional strategies are desirable to improve the charge transfer efficiency and ABPE value.
Recently, engineering single atom catalysts have been widely used to enhance the oxygen evolution reaction performances. Unsaturated coordination environments of single atoms often function as active sites, influencing the catalytic performances 43,44 . However, most single atoms are incorporated into amorphous catalyst layers, and no reports are related to single atom doping semiconductors. For example, atomically dispersed Ni-N 4 sites coordinated with oxygen atom have promoted photogenerated charge separation and thus improved PEC performance of BiVO 4 45 . The single atom catalysts grown on an amorphous support act as the charge transfer layer, and the construction of single-atomic Ni-N 4 -O moiety is located at the interface of photoelectrode/oxygen evolution cocatalyst (OEC). In fact, more serious recombination usually happens in the bulk materials. To this end, it is highly desirable to design single metal atoms doped into photoelectrode to efficiently transfer carrier from the bulks to the photoelectrode/electrolyte interfaces, and improve the OER and PEC performances.
Herein, we develop a versatile strategy to engineer single platinum atom sites coordinated with an oxygen atom (Pt-O) incorporated into single crystalline hematite photoanodes by using 2,2-bipyridine as the ligand to chelate Pt cations, followed by the inert atmosphere treatment to remove the ligand.  Fig. 1a-c). The computed direct band gap of Fe 2 O 3 is approximately 2.0 eV (Fig. 1a), agreeing with the value reported in the literature (2.1 eV) 14,15 . For the NPs Pt/Fe 2 O 3 , partial Pt atoms are bonded to surface oxygens, whilst others exhibit metallic properties, forming the Pt-Pt bonds. The Pt NPs introduce almost continuous states between the band gap of Fe 2 O 3 (Fig. 1b) (Fig. 1c). These trapping states primarily composed of SAs Pt atoms, can trap the photogenerated charge carriers and accelerate the electronhole separation (discussion in the following). Moreover, ELF shows that doping Pt reduces the electron localization in regard to that of the pristine Fe 2 O 3 ( Fig. 1d-f), benefiting to improve electron transport rate. Comparatively, the strong Pt-O and metal-metal interaction exist in NPs Pt/Fe 2 O 3 (Fig. 1e), while the latter vanishes in SAs Pt:Fe 2 O 3 (Fig. 1f). These results confirm that single atomic Pt doping can facilitate the electron  Supplementary Fig. 4d). The formation of Fe 3 O 4 originated from the thermal annealing treatment owing to the oxygen diffusion from the surface to the bulk material. Fe 3 O 4 is not photoactive for photoelectrochemical performance, while it can be assured that it is more conducive than the Fe 2 O 3 , and acts as a conductive layer to transfer charge carrier from the Fe 2 O 3 to the back side 46 . Scanning electron microscopy (SEM) images exhibit one dimensional nanoflakes structure with a length of 1.5-2.5 μm on SAs Pt:Fe 2 O 3 ( Fig. 2b and Supplementary Fig. 5). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images reveal a high degree of single atom dispersion for SAs Pt:Fe 2 O 3 (Fig. 2c-e and Supplementary Fig. 6), where the Pt atoms are uniformly dispersed at the Fe atom positions (Fig. 2f, g). One can see that the edge of nanoflakes presents the defected structure decorated with the isolated Pt atoms ( Fig. 2d and Supplementary Fig. 7). Element mapping shows the homogeneous distribution of Fe, O, and Pt species across the whole nanoflakes ( Fig. 2h), indicative of the presence of single atom ( Supplementary  Fig. 8). Induced coupled plasma-mass spectrometry analysis shows~4 at.% of Pt content on SAs Pt:Fe 2 O 3 . On the contrary, significant aggregation of Pt species with a size of 5-10 nm can be observed on NPs Pt/Fe 2 O 3 (Supplementary Figs. 9-11). The controllable experiment using the same manner without 2,2-bipyridine results in the formation of Pt nanoparticles ( Supplementary Fig. 12). It should be noted that the use of 2,2-bipyridine and low annealing treatment are the critical factors to realize the single atom sites and maintain the dispersion of Pt species. This is a feasible approach of tailoring Pt single atoms doping into single crystalline photoelectrodes.
X-ray photoelectron spectra (XPS) and X-ray absorption fine structure (XAS) spectroscopy were used to probe the local coordination chemistry of the Pt species. The binding energies of Pt 4f 7/2 (72.80) and Pt 4f 5/2 peaks (76.10 eV) on SAs Pt:Fe 2 O 3 are higher than those of metallic Pt 0 and lower than those of Pt 4+ (Fig. 3a), indicating the presence of SAs Pt in hematite. The only Pt 4f peaks representing the metallic Pt appear in NPs:  To investigate the real reaction path of four-electron oxygen evolution reaction (OER) in SAs Pt:Fe 2 O 3 , we calculated the thermodynamic free energy diagrams for OER. The side view structures of *OH, *O, and *OOH absorbed on SAs Pt:Fe 2 O 3 surface are shown in the inset of Fig. 3g. The calculated data in Fig. 3g demonstrate that there are two OER paths (first OER path: in SAs Pt:Fe 2 O 3 surface. Typically, the higher energy barrier of rate-determining steps, the larger overpotentials (η) for OER. As seen in Fig. 3g, we can conclude that the energy barrier of *O intermediate formation in the first OER path (2.07 eV) is lower than that of *OOH intermediate formation in the second OER path (2.48 eV). Thus, the η of first OER path (0.84 V) is lower than that of second OER path (1.25 V). The distinct 0.41 V drop in η shows that the first OER path is the real reaction path in SAs Pt:Fe 2 O 3 , indicating that Pt-O-Fe is the crucial active site which can promote the surface OER kinetics and improve the solar water splitting activity.

Photoelectrochemical performance
The PEC performances were investigated in a three-electrode cell ( Supplementary Fig. 15). Figure 4a shows the photocurrent-potential  (Fig. 4b), suggesting the enhanced surface water oxidation kinetics, especially the surface edge with the substitution of single atomic Pt (Fig. 2d). The improved photocurrent and the reduced V on increase ABPE with a maximum value of 0.51% ( Supplementary Fig. 18), which surpasses the previously reported the highest value of 0.31% 27 . Besides, the current spike on SAs Pt:Fe 2 O 3 (i/i 0 = 0.91) is reduced from the chopped J-V plots ( Supplementary Fig. 19), indicating promoting sluggish water oxidation by single atomic Pt.
To understand the charge character by the influence of SAs Pt, charge separation efficiency (ƞ sep ) and charge transfer efficiency (ƞ tran ) were calculated, and derived from the J-V curves measured in a hole scavenger-containing electrolyte ( Supplementary Fig. 20 (Fig. 4d), higher than that of NPs Pt/Fe 2 O 3 (23.0%), indicating that the fraction of holes reaches to the semiconductor/electrolyte interface without significant recombination in the bulk material. All these results confirm that adjusting the coordination environment via atom-level regulation for PEC water splitting evidently promotes to the charge separation/transfer and realizes efficient energy conversion.
Open circuit potential (OCP) transient decay profile provides an additional information on the photogenerated charge carrier behavior. The OCP in dark is positive and then shifts to the negative direction under illumination ( Supplementary Fig. 21a), which is caused by built-in electric field generated by the photo-generated carriers 27 . SAs Pt:Fe 2 O 3 presents a strikingly accelerated OCP decay as terminating the illumination in relative to NPs Pt/Fe 2 O 3 , representing a large photovoltage generation (ΔOCP = OCP dark -OCP light ). Charge transfer and recombination are principally two competing processes that determine the water oxidation rate on the photoanode surface. To further clarify the charge recombination rate at the semiconductor/ electrolyte junction, the carrier transfer lifetime was calculated from the derived-OCP values, as depicted in Fig. 4e (see details for supporting information). NPs Pt/Fe 2 O 3 displays a carrier lifetime of 11.69 ms when illumination was removed, one time lower than the pristine Fe 2 O 3 (20.32 ms). SAs Pt substitution continues to reduce upon 7.22 ms on Fe 2 O 3 , indicating effective charge separation and fast transfer kinetics. In addition, the incident photon-to-current conversion efficiency (IPCE) on SAs Pt:Fe 2 O 3 is remarkably improved over the entire range of 340-600 nm (Supplementary Fig. 21b). Figure 4f shows the photoelectrochemical impedance spectroscopy (PEIS) for the corresponding photoanodes, and the equivalent circuit was employed to fit the Nyquist plots of PEIS (inset of Fig. 4f) Fig. 22a). The gases produced from the working and counter electrodes present the evolved O 2 and H 2 with a ratio of 2:1 with the Faraday efficiencies of gas productions closed to 100% (Supplementary Fig. 22b). In fact, anion and cations species modulation allow to induce extra electrons near Fe 3+ sites to form Fe 2+ sites, which can enhance the electrical conductivity of Fe 2 O 3 . However, high amount metal doping would principally induce the recombination centers by creating inter-bandgap energy states since the Fe 2+ sites are near to the surface region, causing trapping states and high overpotential for water oxidation. The coordination of single atomic-level Pt substitution for Fe 3+ can avoid the high concentration of doping to a certain degree, which increases the polaron hopping probability, and feasibly enhances the photogenerated charge carrier rate for PEC reaction, consistent with the DFT calculation. On the other hand, atom-level substitution near to the surface would inhibit the interfacial charge recombination, which is reflected by the low V on .
The flat band potentials (E fb ) of SAs Pt:Fe 2 O 3 and NPs Pt/Fe 2 O 3 were conducted by Mott-Schottky measurement. E fb is cathodically shifted since Pt was induced (Fig. 4g), coincided with the shift of V on from the J-V plot (Fig. 4b) Supplementary  Fig. 23), respectively, coherent with the decreased slopes of Mott-Schottky (Fig. 4g). The work functions of the samples were determined by subtracting the cutoff energies (E cutoff ) from the UPS curves (Fig. 4h). The positions of the valence band maxima in regard to the Fermi levels were derived from the onset of valence band photoemission on the low binding energy edges of UPS spectra (inset of Fig. 4h). Based on these results, the determined band positions for the corresponding samples are summarized in Fig. 4i and Supplementary Table 4, according to the literatures reported 3,47 . One can see that tailoring Pt-O sites can influence the band structure, promote the charge separation, and accelerate the photogenerated holes transferring to the semiconductor/electrolyte surface for water oxidation, therefore enhancing PEC performance.

IMPS and TAS analysis for carrier kinetics
Furthermore, we performed intensity modulated photocurrent spectroscopy (IMPS) and transient absorption spectroscopy (TAS) to understand the charge carrier kinetics in various Fe 2 O 3 photoanodes. The semicircle coincides with the competition between interfacial carrier transfer and electron-hole recombination (Supplementary Fig. 24).
The frequency of the maximum imaginary is associated with the sum of charge transfer (k trans ) and recombination (k rec ) rate constants (k trans +k rec ) [48][49][50] . The ratio of k trans /(k trans +k rec ) is taken by comparing the instantaneous photocurrent and steady-state photocurrent. SAs Pt:Fe 2 O 3 exhibits the highest k trans values over the measured potential range (Fig. 5a), indicating that single atoms speed up the charge carrier mobility and quickly transfer from the bulk material to the surface or to the back contact. By contrast, the k rec of SAs Pt:Fe 2 O 3 is lower than that of NPs Pt/Fe 2 O 3 , and almost overlapped with that of the pristine Fe 2 O 3 in the potential of 0.8-1.1 V RHE (Fig. 5b), indicating high charge transfer rate of SAs Pt:Fe 2 O 3 . Further considering surface substitution by SAs Pt, Fermi level pinning effect caused by surface states can be eliminated. With the Pt nanoparticles decorated on the surface, the aggregated particles are inefficient to change the inherent surface trap states possibly owing to the deep-level defects, while atomic Pt substitution does not induce more defects in hematite ( Supplementary Fig. 25). Further on, the high charge transfer efficiency of k trans /(k trans +k rec ) can be obtained on SAs Pt:Fe 2 O 3 , especially in the potential range of 0.8-1.1 V RHE (Fig. 5c), suggesting that the charge recombination is greatly reduced via single atomic bulk and surface doping. In other words, Pt-O coordination favors the photoresponse at low potentials.
TAS can be used to characterize the dynamics of photoinduced charge carrier. The distinct absorption spectra of photoelectrons and holes in photoanode make it possible to monitor the concentration variation of the species, especially the fates of photoholes and photoelectrons on the timescale of picosecond to microseconds. TAS of hematite principally presents two bands, where a bleach band at a probe wavelength of~600 nm is ascribed to trapping photoelectrons, and an absorption band of~700 nm corresponds to photoholes 51,52 . Under OCP condition ( Supplementary Fig. 26), the photogenerated charge carrier dynamics can be associated to the bulk electron-hole recombination since the decay dynamics are faster than the timescale of water oxidation on the surface. From the decay dynamics photoinduced absorption (Fig. 5d-f), the carrier dynamics for the samples was calculated by fitting the kinetics traces at 600 nm and 700 nm (Supplementary Table 5 shows the long lifetimes at 600 and 700 nm relative to NPs Pt/Fe 2 O 3 and Fe 2 O 3 , indicative of the long-lived photoholes for water oxidation. The pristine Fe 2 O 3 exhibits a fast decay among all samples due to the serious electron-hole recombination. These provide strong evidence that the charge recombination kinetics can be greatly suppressed by SAs Pt doping, which favors to the water photooxidation of hematite under the present experimental conditions. We next carried out the nonadiabatic (NA) molecular dynamics with decoherence-induced surface hopping (DISH) approach to analysis the charge carrier dynamics of hematite, and the simulated data are shown in Fig. 5g-i. Fitting the curves to an exponent, P t ð Þ = expðÀ t τ Þ, we obtained the charge trapping, detrapping, transfer, and recombination time scales (Supplementary Table 6). In general, electron-hole recombination results in short carrier lifetime and reduces OER performance. In the pristine Fe 2 O 3 system, the photogeneration charge carriers occur the detrimental recombination process (Fig. 5g). Doping the Fe 2 O 3 with Pt NPs (Fig. 5h), free carrier recombination process is inhibited compared to the Fe 2 O 3 system, and the electron transfer between Fe 2 O 3 and NPs Pt becomes the main process. The fast charge transfer promotes efficient charge separation and prolongs the carrier lifetime. Changing the NPs Pt to SAs Pt (Fig. 5i), the occurrence of free carrier recombination is further reduced compared to NPs Pt/Fe 2 O 3 system, and the charge carriers mainly take place in trapping and detrapping processes. The charge trapping and detrapping are in dynamic equilibrium, which extends the carrier lifetime and improves the OER performance of Fe 2 O 3 comparable to NPs Pt doping, demonstrating the above experimental results.

Further PEC performance enhancement via surface oxygen vacancies
Benefited from the role of single atom substitution, we further used plasma etching treatment to produce surface oxygen vacancies (O V ) and accelerate the charge transfer on SAs Pt:Fe 2 O 3 (remarked as SAs Pt:Fe 2 O 3 -O V ). No evident morphology change can be discerned for the treated sample ( Supplementary Fig. 27). Single atoms are still retained from HAADF-STEM observation (Fig. 6a). Surface vacancies can be viewed as marked by the circle (Fig. 6b), accompanied by the irregular surface boundary originated from plasma treatment (Supplementary Fig. 28). Moreover, SAs Pt:Fe 2 O 3 -Ov maintains two peaks at 1.96 Å and 2.86 Å in the Pt L-edge EXAFS spectrum (Fig. 6c), representing the coordination of Pt-O and Pt-O-Fe (Supplementary Fig. 29). The slight decrease in peak intensity on SAs Pt:Fe 2 O 3 -O V can be observed compared with SAs Pt:Fe 2 O 3 ( Supplementary Fig. 30 Table 7). The fitting results of χ(R) and χ(k) space spectra with reasonable R-factor quantitatively support the local atomic structure and coordination numbers information. O 1s XPS spectrum shows that the lattice oxygen is shifted to higher binding energy by 180 mV for SAs Pt:Fe 2 O 3 -O V , while Pt 4f XPS spectrum is negatively shifted after plasma treatment (Supplementary Fig. 31). This means that SAs Pt in Fe 2 O 3 -O V are electron-richer compared with SAs Pt:Fe 2 O 3 , and more charge can be transferred from Fe 2 O 3 to Pt. Additionally, Pt content on the top surface has decreased from 9.96 (without plasma treatment) to 6.97 at.% (Supplementary Table 8). This can be originated from the generated surface O V , which anchors the Pt single atom to stabilize it.
The PEC performance of SAs Pt:Fe 2 O 3 -O V is further improved after optimization ( Supplementary Fig. S32), in which the photocurrents reach to 3.65 and 5.30 mA cm -2 at 1.23 and 1.5 V RHE with a maximum ABPE value of 0.68% (Fig. 6d, e). This is more than double the previously reported highest value of the doped Fe 2 O 3 -based photoanodes  Table 1), and even superior to the cocatalysts decorated Fe 2 O 3 -based photoanodes (Supplementary Table 9). Meanwhile, V on shifts to the negative direction with ΔV on = 5 mV, in line with the negative shift of the flat band potential on SAs Pt:Fe 2 O 3 -O V deduced from the Mott-Schottky plot (Supplementary Fig. 33). This is currently known as one of the best performances in terms of photocurrent and V on for elemental doping hematite semiconductors. Moreover, the electrochemical specific surface area of SAs Pt:Fe 2 O 3 -O V is evidently increased, almost 4 times higher than that of the SAs Pt:Fe 2 O 3 (Supplementary Fig. 34). There is no notable enhancement on the charge separation between SAs Pt:Fe 2 O 3 -O V and SAs Pt:Fe 2 O 3 , but the charge transfer efficiency of SAs Pt:Fe 2 O 3 -O V highly increases ( Fig. 6g and Supplementary Fig. 35), supporting the charge transfer influenced by the oxygen vacancies. The charge carrier derived from OCP displays that SAs Pt:Fe 2 O 3 -O V has faster photoresponse ( Supplementary  Fig. S36), along with higher k trans value, lower k rec values ( Fig. 6h and Supplementary Fig. 37), and long charge carrier lifetime (Fig. 6f, i) comparable with non-treated SAs Pt:Fe 2 O 3 (Supplementary Fig. 38). The surface recombination is generally happened which affects the oxygen evolution reaction and PEC activity of the photoelectrode. Here the surface oxygen vacancies induced can serve as surface states below the conduction band minimum that boost the charge carrier transfer. The defects raise the flat band potential away from the redox potential of O 2 /H 2 O and increase the upward bending of the hematite band edge.
This can facilitate the charge transfer from the Fe 2 O 3 surface to the electrolyte. In other words, surface oxygen vacancies are conductive to a reduced interfacial charge-transfer barrier and surface trapping states. PDOS of SAs Pt:Fe 2 O 3 -O V demonstrates that oxygen vacancies can eliminate the trap states between the band gap of Fe 2 O 3 (Supplementary Fig. 39a, b), reducing the charge recombination. Also, ELF (Supplementary Fig. 39c) shows that oxygen vacancies noticeably reduce electron localization and electron transport resistance, which act as active sites for chemisorption of the intermediates, and improve the surface water ability. All these results demonstrate that the synergistic effect of single atom coordination and surface oxygen vacancies are responsible for the improved charge transfer and the suppressed photogenerated charge recombination, boosting water oxidation ability.
Based on the calculation and experimental results above, a mechanism is proposed to clarify the improved charge carrier kinetics for SAs Pt decorated on the defected Fe 2 O 3 . The influence of charge transfer and recombination is schematically illustrated in Supplementary Fig. 40. When Fe 2 O 3 is exposed to light to generate photogenerated carriers, most of the photo-generated carriers would be recombined instantaneously before arriving to the surface for water oxidation reaction owing to the short hole diffusion length. Few separated photogenerated electrons and holes in the conduction band and the valence band are transferred to the corresponding surface to participate in oxygen and hydrogen evolution reactions, causing the poor performance. When Pt species were incorporated to Fe 2 O 3 , the mobility of excited carrier can be accelerated, thereby improving the carrier transfer efficiency. In contrast to the traditional element doping, single atoms coordination is in favor of improving the charge separation efficiency and prolonging the charge carrier lifetime by the shift of the photogenerated holes towards the photoanode/electrolyte interface and of the electrons to the back side. On the other hand, single atom-level substitution can efficiently suppress the deeplevel defects in hematite relative to the nanoparticle/cluster-level doping, in which the latter with aggregated defects has a distribution along the photoanode. Further surface oxygen vacancies were produced in the material, thereby surface oxygen vacancies and reduced Fe 2+ species within the nanoflakes following the doping reaction mechanism. In addition, one-dimensional single-crystalline nanoflakes structure with enough light harvesting could facilitate the charge collection efficiency.

Discussion
In summary, single platinum atom doped into Fe 2 O 3 photoanode has been successfully synthesized by using 2,2-bipyridine as the ligand to chelate Pt cations, followed by the inert atmosphere treatment.

Material preparation
Iron foils (Alfa Aesar, 0.25 mm thick, 99.99%) with a size of 1 cm 2 were degreased by ultrasonic treatment in acetone and ethanol for 10 min, and then dried in a nitrogen stream. To prepare the α-Fe 2 O 3 nanoflakes grown on iron substrate, iron foils were thermally annealed in a furnace (HF-Kejing Furnace, KSL-1100X) at 400°C in air at a heating rate of 10°C min −1 and kept at the required temperature for 3~4 h. The entire sample surface became nanoflakes during the thermal annealing for the following treatment. Hexachloroplatinic acid hexahydrate (5 mmol) dissolved in ethanol was mixed with 2,2-bipyridine in a molar ratio of 1:1, 1:3, and 1:6 for 10 min The α-Fe 2 O 3 nanoflakes were immersed into the above solution for different times (5 min, 15 min, 25 min, and 35 min), following by drying overnight in a vacuum oven at 60°C. Next, the samples were annealing in Ar at 330°C and 400°C for 100 min for the synthesis of SAs Pt:

Photoelectrochemical characterization
Photoelectrochemical measurements were measured in a standard three-electrode system with a CHI 760D electrochemical analyzer. The light source was used the simulated AM 1.5 G (100 mW cm −2 ) sunlight. The solar simulator used for PEC measurement was equipped with a total-reflection mirror and AM 1.5G fitter, and the spectrum was measured as shown in Supplementary Fig. 41. 1 M Potassium hydroxide (KOH, pH = 14) was used as an electrolyte. The prepared sample, Pt foil, and Ag/AgCl were used as the working electrode, counter electrode, and reference electrode, respectively. Photocurrent vs voltage (I-V) curves were recorded by scanning the potential from −0.5 to 0.65 V Ag/ AgCl with a rate of 10 mV s −1 . The measured potential was converted into a potential with respect to a reversible hydrogen electrode (RHE). There is no iR correction performed in the experiment. Electrochemical impedance spectroscopy (EIS) was performed at 1.23 V RHE and a small AC amplitude of 10 mV in the frequency range of 10 −2 -10 5 Hz under AM 1.5 G illumination.
Applied bias photon-to-current efficiency (ABPE) can be calculated using the following equation of ABPE(%) = , in which J is the photocurrent density (mA cm −2 ) obtained from AM 1.5G illumination, V b refers to the applied bias potential versus RHE, and P total is the total light intensity of AM

Material characterization
The crystalline structure and composition were performed by X-ray diffraction analysis (XRD, Rigaku RINT-2000, Cu Kα radiation at 40 kV and 40 mA) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, Thermo Fisher Scientific). The elemental contents were tested by inductively coupled plasma optical emission spectroscopy (ICP-OES-720ES, Agilent, USA). The morphology was observed using field emission scanning electron microscopy (FE-SEM, Supra 55, Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) systems. The cross-sectional SEM image was taken by argon-ion milling machine (GATAN, ILION693) with a voltage of 5 kV, following with SEM observation. The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy was performed using JEM-ARM200F. UV-visible diffuse reflectance spectra were implemented on a UV-2600 (Shimadzu) spectrometer using BaSO 4 as the reference. Photoluminescence (PL) spectra were performed on a HORIBA Fluoromax-4 (HORIBA JY, HORIBA Fluoromax-4, USA) under laser excitation at 350 nm. The electron paramagnetic resonance (EPR) measurements were recorded using a JES-FA200 spectrometer at low temperature (−150°C).

Charge carrier kinetics measurements
Intensity modulated photocurrent spectroscopy (IMPS) was performed with a potentiostat (PGSTAT302N, Metrohm), an impedance analyser (FRA32M, Metrohm), and a light-emitting diode (LED) driver kit (Metrohm) that drove illumination of 420 nm power UV LED in 1 M KOH at different voltages (0.8 V RHE , 0.9 V RHE , 1.0 V RHE , 1.1 V RHE , 1.2 V RHE , 1.3 V RHE , 1.4 V RHE , and 1.5 V RHE ). The LED intensity was 5.5 mW cm −2 , and it was modulated by 10% in the range of 10 kHz-0.1 Hz. Transient absorption spectroscopy (TAS) measurements were performed on a Helios (Ultrafast systems) spectrometers using a regeneratively amplified femtosecond Ti:sapphire laser system (Spitfire Pro-F1KXP, Spectra-Physics; frequency, 1 kHz; max pulse energy,~8 mJ; pulse width, 120 fs) at room temperature. For the TAS sample preparation of the pristine hematite, ten pieces of Fe 2 O 3 nanoflakes with the same experimental condition were scratched from the iron foils into a 5 ml sample storage using a plastic dropper. Then, 3 ml deionized water was added into the storage, and was ultrasonic treated for 30 min to maintain high dispersion of the sample. For comparison, we conducted the TAS measurement of the deionized water to eliminate its disturbance during the measurement. The TAS sample preparations of NPs Pt/Fe 2 O 3 and SAs Pt:Fe 2 O 3 were used the same treatment of the pristine Fe 2 O 3 . The data were analyzed through commercial software (Surface Xplorer, Ultrafast Systems). An individual three-exponential decay model was used to calculate the fits of the decay. The amplitude weighted average lifetime (τ av ) can be fitted using the following equation of τ av= SUM½Ai × τi SUM½Ai , where A i is the amplitude of the component with lifetime (τ i ), and τ i is the amplitude weighted lifetime.

Simulation methodology
Geometry optimization, electronic structure, adiabatic molecular dynamic (MD), and nonadiabatic (NA) coupling calculations were performed using Vienna ab initio simulation package (VASP) software 53 . The electron exchange-correlation and electron-ion core interactions were treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) 54 and projector-augmented-wave (PAW) 55 approaches, respectively. Typically, the Fe 3d strong correlated electrons cannot be well-described by the standard DFT method. Therefore, to correct the strongly correlated electronic nature of Fe 3d-electrons, we applied the on-site Coulomb correction (U = 5 eV for Fe 3d orbitals) to accurately describe the band gap of Fe 2 O 3 (~2.0 eV), which agrees well with the data (2.1 eV) calculated using the same functional method 56 . The plane wave cutoff energy was set to 500 eV. The weak van der Waals interactions were described with the Grimme DFT-D3 methdsh 57 . The geometry optimizations were carried out at Γpoint because a large supercell was used. The electronic structure calculations were performed on 2 × 2 × 1 grid for k-point sampling. After the geometry optimization, all systems were heated to 300 K by repeated velocity scaling. Then, 4 ps microcanonical ensemble adiabatic MD trajectories were obtained. The nonadiabatic molecular dynamic (NAMD) simulations were calculated with the decoherenceinduced surface hopping (DISH) method 58 implemented within the time-dependent Kohn-Sham density functional framework [59][60][61] , which were performed using the PYthon eXtension for Ab Initio Dynamics (PYXAID) code 62,63 .
The four-electron OER occurs based on Equations (1) The Gibbs free energy G was calculated with the followed equation of G = E + E ZPE À TS À eU. E, E ZPE , S, U, and T correspond single point energy, zero-point energy, entropy, potential versus standard hydrogen electrode, and temperature (298.15 K), respectively. The overpotential (η) toward OER was computed using the equation (U = 0 V) of η = max ΔG1,ΔG2,ΔG3,ΔG4 f g e À 1:23: The ΔG1,ΔG2,ΔG3, and ΔG4 represent the Gibbs free energy difference for elementary reactions.